Process for forming metal coatings



June 10, 1969 A. L. BALD! 3,449,159

PROCESS FOR FORMING METAL COATINGS Filed Feb. 14, 1966 Sheet of 2ATTORNEYS June 10, 1969 A. L. BALDI 3,449,159

PROCESS FOR FQRMING METAL COATINGS Filed Feb. 14, 1966 Sheet 3 of 2 INVENTORL ATTORNEYS United States Patent 3,449,159 PROCESS FOR FORMINGMETAL COATINGS Alfonso L. Baldi, Drexel Hill, Pa., assignor to AlloySurfaces Company, Inc., Wilmington, Del., a corporation of DelawareFiled Feb. 14, 1966, Ser. No. 527,148 Int. Cl. C23c 13/02 U.S. Cl.117107.2 17 Claims ABSTRACT OF THE DISCLOSURE The present invention insummary relates to the creation of deposits on the surface of ferrousmetal containing carbon. In the broadest aspect of the invention theferrous metal is brought into contact with a granular oxide of a metalwhich forms a carbide having a substantial negative free energy offormation, in the presence of a reducing gas such as hydrogen at atemperature of 1,500 to 2,000 F. This creates a deposit of extremethinness on the surface of the ferrous metal, the deposit predisposingthe surface to form a chromized layer which will be smooth rather thanrough. The ferrous metal may be steel which has been decarburized andhas a carbon content between 0.001 and 0.010%. It may also be a steelhaving a carbon content between 0.001 and 0.20% which has beenstabilized by introducing titanium, columbium, vanadium or tungsten inan appropriate ratio to the carbon content.

Further in summary, the surface of the stabilized steel having a carboncontent between 0.001 and 0.20% can be predisposed to take a smoothchromized layer, without the previously mentioned annealing, in thepresence of a granular oxide, by regulating the content of stabilizerwithin a narrow low range with respect to the carbon content.

DISCLOSURE OF INVENTION A purpose of the invention is to permit theformation of thin deposits on ferrous metal containing carbon which willinclude a carbide of a metal which forms a carbide having a substantialnegative free energy of formation.

A further purpose is to create a preferential layer on ferrous metalwhich will predispose it to form a smooth chromized layer by depositionrather than etching and substitution.

A further purpose is to provide a surface on ferrous metal containingcarbon by annealing at a temperature of 1,500 to 2,000 F. in contactwith a granular oxide of a metal forming a carbide with substantialnegative free energy of formation, so as to predispose the surface toform a smooth chromized layer.

A further purpose is to produce a greater case depth by chromizing in agiven time or to reduce the time required to produce a given case depth.

A further purpose is to reduce the consumption of chromium in chromizingand minimizing the extent of contamination of the source of chromiumwith iron during chromizing.

A further purpose is to obtain a desposition reaction rather than aninterchange reaction in chromizing.

A further purpose is to minimize the extent of adherence of the sourceof chromium to chromized work in pack or contact chromizing, reducingthe labor required to clean or clean and polish the chromized productafter chromizing.

A further purpose is to accomplish annealing and the creation of a thindeposit on ferrous metal work by blowing the granular oxide into contactwith the work during annealing and permissibly while the work isadvancing through an annealing chamber.

Further purposes appear in the specification and in the claims.

In the drawings I have chosen to illustrate a few only of the numerousembodiments in which the invention may appear, selecting the forms shownfrom the standpoints of convenience in illustration, satisfactoryoperation and clear demonstration of the principles involved.

FIGURE 1 is a diagrammatic central vertical section of an open coilannealing furnace for use in the present invention.

FIGURE 2 is a diagrammatic vertical section of a continuous annealingfurnace according to the present invention.

In the prior art, both in gaseous chromizing by circulating ahalogen-containing gas and a carrier gas through a source of chromiumsuch as ferrochrome and then in contact with the work according toSamuel and Bell U.S. Patent No. 3,222,212, granted December 7, 1965, forProcess of Chromizing, and also according to the pack technique in whicha source of chromium such as ferrochrome, coated or impregnated with asuitable halogen chromizing catalyst such as bromine, chlorine, fluorineor iodine, suitably in the form of a chromous halide, is brought intocontact with or adjacent to ferrous metal work, the results haveremained up to the present time rather unpredictable. Two lots of steelwhich superficially seem to be the same produce quite different resultsfrom the standpoint of the chromized case, even though the chromizingtechnique is the same. One lot of steel undergoes a chromizinginterchange reaction, iron leaving its surface in the form of ferroushalide, and depositing on the source of chromium, and chromium formingchromous halide and interchanging with the iron of the work, producingon the work surface a very bright but very rough chromized case. In thisinstance slight dimensional loss in the work or no dimensional gainoccurs in chromizing. In this type of chromizing I find that thequantity of deposition in a given time is rather moderate. There is agreat tendency for the ferrochrome or other source of chromium to adhereto the work if a chromizing pack is in contact with the work. As aconsequence, a great deal of labor must be expended by way of cleaning01f adhering particles and also burnishing the product to remove theroughness.

Furthermore, in this type of chromizing the source of chromium becomescontaminated with iron more extensively than in the other type to bediscussed, and must be replaced when iron contamination is excessive.

Unpredictably in the prior art some steel specimens, on the other hand,produce dull, smooth chromized layers to which the particles offerrochrome, or other pack, if present, have less tendency to adhere.They can be very quickly and inexpensively brightened by burnishing.Instead of an interchange reaction, in this instance the chromium simplydeposits and there is very little loss of iron so that the source ofchromium is kept free from contamination. Instead of losing dimension,or maintaining dimension, as in an interchange or interchange plusetching reaction, there is simply a dimensional change approximatelyequal to one-half the case depth.

I have conducted an extensive experimental program in an effort todetermine the factors which lead to one type of chromizing rather thananother.

ANNEALING I first attempted to determine whether grain size or shape isa factor. I was unable to find any correlation between grain size andgrain characteristics in reasonably fine grained steels which wouldaccount for the two different types of chromizing.

As a result of annealing experiments, however, I discovered with somesurprise that when I annealed relatively low carbon steels in thepresence of certain granular oxides, and in reducing atmosphere,notwithstanding the free energy considerations suggesting the oxideswere of great stability, something happened to the surface of the steelwhich changed its characteristics and made it uniformly form smooth,dull chromized layers in later chromizing, rather than rough, brightchromized layers. Furthermore, this led to a deposition rather than aninterchange reaction, with dimensional growth rather than loss or nochange in dimension, and freedom from tendency of the pack, if any, toadhere to the chromized case.

When reference is made to granular oxide, it is meant that the oxide isfinely divided, in the form of individual particles, which willpreferably be a powder. The oxide should be at least fine enough to passthrough a screen of 20 mesh per linear inch, and preferably fine enoughto pass through a screen of 60 mesh per linear inch, although in mostcases a much finer particle size of oxide will be used, suitably through100 mesh per linear inch, or preferably through 325 mesh per linearinch. When mesh are referred to herein it is intended to designate Tylerstandard mesh per linear inch. When the granular material is required tobe circulated by the gas to bring it into contact with the work, it willof course be understood that it should be fine enough so that the gascan carry it under the particular gas velocity conditions, for examplethrough 100 mesh per linear inch.

I first found this effect when annealing in a hydrogen atmosphere with avery stable oxide, alumina and later by extending my experiments I foundthat a similar effect occurs when annealing in the presence of othergranular oxides which have a substantial negative free energy offormation of the corresponding carbide.

It certainly would not be supposed that hydrogen would reduce alumina toform aluminum, but contrary to expectation I find that a small butimportant quantity of aluminum is reduced under these conditions. Itappears to form on the surface of the iron a layer of aluminum carbideby reaction with carbon in the steel. This definitely improves thesurface and makes it harder and more favorable to receive chromizeddeposits.

Trying other oxides, I find that similar efiects occur for example whenthe ferrous metal is annealed in the presence of titania or chromicoxide.

It seems that the effect is attributable to the fact that the particularelement forms a carbide which has a high or substantial negative freeenergy of formation, and therefore is highly stable when applied to thesurface of the steel. Elements of this class which can be used as oxidesare aluminum, boron, calcium, columbium (niobium), chromium, molybdenum,silicon, strontium, tantalum, thorium, titanium, tungsten, uranium,vanadium, and zirconium.

The temperature of annealing will suitably be in the range of 1,500 to2,000 F., preferably 1,700 to 1,850 F.

The oxides and the hydrogen or other reducing gas are preferably dry.

The gas can simply be hydrogen or cracked ammonia, or a mixture ofnitrogen, carbon monoxide and hydrogen. The gas is preferably flowed orcirculated through the annealing chamber and can conveniently be burnedat an exit point. Hydrogen is decidedly preferable because of its strongreducing action. The gas is preferably substantially free from elementalhalogen and from hydrogen halide.

The granular oxide can be packed around the work, or it can be blown forexample by the reducing gas into contact with the work. If the steelwork is progressing through the annealing chamber, the gas can bring theoxide against the work.

FERROUS METAL WORK The invention is applicable to two types ofmaterials, one a low carbon steel which is of such low carbon contentthat it may be considered to be decarburized since this is the normalway that it is produced. In this case the carbon content is between0.001 and 0.01%, the maximum carbon content usually being 0.005%, thebalance being essentially the usual metalloids such as manganese,phosphorus, sulphur and iron.

The widest application of the invention, however, is to low carbon steelhaving a range of carbon content between 0.001 and 0.20% and stabilizedby the presence of one of the metals titanium, columbium, vanadium ortungsten or a combination thereof. The balance of this steel isessentially the usual metalloids and iron.

These steels are not essentially alloy steels, but they may have alloyother than carbon and stabilizer totalling 5% or less by weight.

A typical steel of this kind, stabilized by titanium, is used innumerous experiments referred to below and has an analysis as follows:

Percent Carbon 0.05 Titanium 0.30 to 0.40 Manganese 0.30 Phosphorus 0.01Sulphur 0.02 Copper 0.04 Aluminum 0.02 Silicon 0.03

The stabilized steels as conventionally manufactured have a lower limitof the stabilizing element content at the stoichiometric ratio and rangein ratio of the weight of the stabilizing element to the weight of thecarbon as follows:

Titanium Between 4 and 12 to 1. Columbium Between 8 and 20 to 1.Vanadium Between 4 and 12 to 1. Tungsten Between 15 and 50 to 1.

The reason for the broad spread in the case of tungsten is that tungstenforms two different carbides.

As explained later in the present specification, however, there will insome cases be advantages in using lower ratios of stabilizer to carbonand these should be included.

EFFECT OF STABILIZER RATIO I have discovered that in the case of the lowcarbon stabilized steels, there is another way to create a favorablesurface condition for formation of smooth chromized layers withdeposition rather than interchange. This technique can be used insteadof or in addition to annealing.

Some of the stabilized low carbon steel specimens produce smoothdeposition chromized layers without an nealing, while others if notannealed in the manner explained above produce bright, rough interchangechromized layers. By carefully determining the ratio of stabilizer tocarbon in the particular steel, I find that a high ratio of stabilizerto carbon favors the formation of bright, rough interchange chromizedlayers, while a low ratio of stabilizer to carbon favors the formationof smooth, dull deposition chromized cases. This seems to confirm theidea that if there is a great content of stabilizer the carbon is notfree to produce carbide at the surface which can promote the formationof a smooth, dull deposition chromized case.

Furthermore, it appears that use of stabilizer ratios to carbon whichare less than the stoichiometric theoretical, definitely favors theformation of smooth, dull deposition chromized cases.

In the instance of titanium stabilized steels, in order to have afavorable result in promoting a smooth, dull deposition chromized casethe ratio of titanium to carbon should be between 2.5 and 5.75 to 1. Themost eifective ratio is between 3 and 5.5 to l.

With other stabilizers the ratios should be as follows:

The chromizing may be pack chromizing, in which the ferrochrome ormetallic chromium or other source of chromium will be adjacent to or incontact with the ferrous metal work. The procedure employed may conformto Baldi and Nice U.S. patent application Serial No. 488,711, filedSept. 2, 1965, for chromizing and Priming, now U.S. Patent No.3,375,128; Samuel U.S. Patent 2,899,322, granted Aug. 11, 1959, forChromizing Method and Composition; Samuel and Bell U.S. Patent2,921,877, granted Jan. 19, 1960, for Process of chromizing AirHardening Tool Steel; Samuel U.S. Patent 2,851,375, granted Sept. 9,1958, for Ductile chromizing; Samuel U.S. Patent 2,811,460, granted Oct.29, 1957, for Process of Chromizing; Samuel U.S. Patent 2,825,658,granted Mar. 4, 1958, for Method of chromizing; and Samuel U.S. Patent2,811,466, granted Oct. 29, 1957, for Process of Chromizing.

When pack chromizing is referred to in the experiments, the followingtechnique has been used for the temperature and time and with theparticular specimens referred to in the particular experiment:

A inch diameter Inconel retort 21 inches high was used. A 1 inch bed ofporous ferrochrome containing 68 percent chromium and 32 percent iron byweight which in turn contained in its pores 1 percent by weight ofchromous chloride was placed on the bottom of the retort. Five grams ofaluminum chloride was spread on top of the 1 inch bed of primedferrochrome. Next the specimens to be chromized were packed in the restof the retort in contact with the ferrochrome primed with the 1 percentchromous chloride. The specimens were well surrounded by the ferrochromeand they were not permitted to contact each other so as to preventshielding. The top of the retort containing a bleeder tube was sealed tothe body by a rubber gasket. The head was cooled by flowing waterthrough it. The retort but not the head was next placed in a gas firedfurnace and the temperature increased up to the chromizing temperaturespecified. This temperature is usually between 1700 F. and 1900 F.During the heat-up the aluminum chloride forced out of the retort anyexisting moisture or air. The evolved gases were readily observed byplacing the bleeder tube in a container of water. After the operatingtemperature was attained and all of the contaminates such as oxygen,air, and moisture were out of the retort, a slight vacuum wasencountered as indicated by a slight withdrawal of water (about 1 to 2inches) in the glass bleeder tube. At this point, the retort was pinchedoff by clamping a rubber tube at the exit of the retort. In this mannerthe chromizing react-ion took place under a slight vacuum. After holdinga temperature for the specified time period (normally 2 to 10 hours) thefurnace was turned off and the retort permitted to cool under the slightvacuum. After the internal contents of the retort had attained roomtemperature, the retort was opened and the parts unloaded and washed forinspection.

The invention is also applicable to gas chromizing, of the typedisclosed in Samuel and Bell U.S. Patent 3,222,- 212, granted Dec. 7,1965, for Process of chromizing, in which a carrier gas such as hydrogenand a halogencontaining gas such as hydrogen bromide, hydrogen chloride,bromine or chlorine is circulated in a furnace to pass the gas under theaction of a pump or fan continuously through a source of chromium toform chromous halide and then in contact with the work such as an opencoil of steel to be chromized to form a chromized layer.

When gas chromizing is referred to in later experiments the particulartechnique was as follows, applying it for the time and at thetemperature and on the particular work referred to in the particularexperiment: five baskets three feet in diameter and 8 inches high werestacked on top of one another. A perforated sheet consisting of amultitude of A inch diameter holes was supported by a collar at thebottom of each basket. A 32 Tyler mesh sieve was placed on top of theperforated sheet. A 1 inch bed of porous ferrochrome containing 68%chromium and 32% iron by weight was then placed on top of the screen.Note that this ferrochrome was not primed with any halide. A 32 Tylermesh screen was then placed on top of the bed of ferrochrome. The partsof specimens to be processed Were placed in the 4 lower baskets on topof the screen and not touching the ferrochrome. An Inconel retort wasplaced over the baskets and sealed to the base by a rubber gasket-flangeassembly. The base was water cooled. A 1 inch tube extended through thebase into the retort so that various atmosphere gases and halides couldbe injected at this point. Another 1 inch tube was placed at from theinlet tube. This tube acted as the outlet and extended from within theretort through the base to the outside. A fan was located at the baseand its purpose was to pull gases entering in the inlet tube around theoutside of the baskets and through the inside of the baskets so as tohave a continuous recirculation during the processing cycle. Theprocessing cycle consists of purging out the retort with nitrogen at aflow of 350 c.f.h. After one hour of purging, the furnace was fired up(gas fired furnace) and the nitrogen purge was continued until atemperature of approximately 900 F. was attained inside the retort. Atthis temperature a flow of 350 c.f.h. of hydrogen was injected into theretort and the nitrogen flow stopped. After the chromizing temperaturewas reached (1700 to 1900 F.), the hydrogen flow was lowered to about 10c.f.h. Twenty c.f.h. of argon and 4 c.f.h. of hydrogen bromide were alsoinjected into the retort along with the hydrogen. The hydrogen bromidefirst contacted the bed of ferrochrome and reacted with it to formchromous bromide. The chromous bromide then reacted with the steelspecimens to carry out the chromizing reaction. The reaction productsthen contacted the bed of ferrochrome in the second basket to again formchromous bromide which in turn reacted with the steel specimens toperfect the chromizing reaction. The gases were recirculated around theretort and thereby maintaining a good chrome potential through thechromizing reaction. An amount of gas mixture was constantly beingejected through the exit pipe in order to maintain a loW dew point inthe system and to permit continuous generation of chromous bromide bythe introduction of hydrogen bromide at the inlet tube. The chromizingcycle can be anywhere from two to twenty-four hours to provide the casedepth required. At the end of the chromizing cycle, the hydrogen bromideand argon flow were stopped and the hydrogen flow increased to 350 c.f.hto purge out the retort. When the temperature attained about 900 F., 350c.f.h. of nitrogen were injected into retort and the hydrogen flowstopped. The flow of nitrogen was continued until the internaltemperature of the retort was below 200 F. The retort was then removedand the parts unloaded from the baskets, washed, and inspected.

THEORY In the case of the annealing technique followed by either packchromizing or gaseous chromizing, it would seem that several differenteffects are taking place during the annealing:

1) The steel is to a considerable extent decarburized by the action ofthe hydrogen or other reducing gas.

(2) A small but appreciable quantity of the metal is reduced from theoxide in contact with the work and is deposited on the surface of thework. This is very strange because the free energy of formation of theoxide is greater than that of the carbide, but the fact of reduction 7is clearly established. It seems that the metal oxide may initiallyfunction as a catalyst to promote reduction before it participates inthe reaction.

(3) This deposited metal, which may in a particular case be titanium,forms surface carbide, say titanium carbide, which has a substantialnegative free energy of formation and will tend to be stable.

(4) In the latter chromizing this metal carbide tends to reduce theetching or attack by the halogen on the surface of the work and preventthe formation of iron halide which would otherwise take place and wouldencourage an interchange reaction.

(5) The titanium carbide or other carbide of substantial negative freeenergy of formation at the surface does not readily react with thereducing gas to permit decarburization and therefore it tends to holdcarbon which would otherwise be removed by decarburization, creating acarbon-rich area at the surface.

(6) From this carbon-rich area at the surface carbon is available todiffuse into the case, making the case harder than it otherwise would beand preventing sticking of the source of chromium to the surface in packchromizing.

(7) Since more of the reaction conveys chromium to the work and less ofthe reaction conveys iron halide away from the work, there is a thickercase deposited in a given time, and there is less contamination of theferrochrome by iron. Since there is little etching and specially lesspreferential etching, there is greater smoothness of the chromizedsurface.

These theoretical considerations, while not essential to the disclosure,seem to be factual in view of the increased weight of the specimen afterannealing, and the buildup of carbon in the case by this procedure.

Where the stabilizer ratio to carbon alone is the controlling factor, asin titanium stabilized steel without annealing, it would appear that theratios of free energy of various compounds may be controlling. Forexample, let us consider the free energy of formation of varioustitanium compounds at room temperature and at 1,800 F.

TiC

Case Core grain structure depth, Gain or loss in Weight Identificationbefore chromizing Surface after chromizing mlls after chromizingReaction mechanism Untreated 3748 Elongated Rough and brig g -l qit Etc'ng plus lnterchange. Untreated 488772. d0 0 gr-lsqin-.. Do. Annealed3748.... Equiaxed Smooth and dull. -1 gnsq. ft.--. Deposition. Annealed488772 do do 3 gL/SQ. it Do.

creased the conversion to titanium tetrachloride is less feasible. Thisis borne out by Example 14, in which one lot of low carbon titaniumbearing steel was chromized in three different conditions, with a scaledor oxidized surface, a pickled surface and a sand blasting surface. Themost uniform deposit was obtained on the scaled surface, which of coursehad at the surface the highest amount of titanium dioxide.

Example 1 Samples of titanium stabilized steel having an analysis as setforth above taken from two lots (3748 and 48872) which gave roughchromized surfaces in gas chromizing as above explained were obtained inthe form delivered by the steel mill and treated as follows:

(1) Degreased with a solvent.

(2) Placed in a retort having a semi-tight lid packed all around incontact with the steel with powdered alumina which had previously beencalcined at a temperature of about 1900 F. The retort was placed in anannealing furnace with hydrogen flowing through the retort and and outthe opening in the lid and displacing all air from the retort, and theretort and contents were heated to 1,800 F. and held at this temperaturein contact with the flowing hydrogen for five hours. The hydrogen had apurity of 99.9% and it had a dew point rating of about minus 100 F.

After completion of the hydrogen annealing as above described, theretort and its contents still in contact with the hydrogen was cooled toroom temperature, and opened and the steel specimens were removed.

The specimens as annealed above in hydrogen in contact with alumina werethen chromized in contact with a pack of porous ferrochrome which hadbee impregnated with anhydrous magnesium chloride according to theprocedure in my copending application, Serial No. 488,711, filed Sept.2, 1965, for Chromizing and Priming, now US. Patent No. 3,375,128. In asealed retort, in which the air was excluded by the specimens as treatedabove and other specimens from the same steel lots which had merely beendegreased were heated to 1,775 F. and then held at this temperature incontact with the impregnated ferrochrome for six hours and then cooledin the retort while it remained in the furnace.

The results are summarized in the following table:

It will be noted that the free energy of formation of titanium dioxideand titanium tetrachloride is very high. Since titanium tetrachloride isvery volatile and has a high vapor pressure, in a chloride atmosphereduring chromizing it is very likely to form and remove titanium from themetal interface if titanium remains there as metallic titanium.Accordingly, this might lead to etching and formation of rough surfacein chromizing a titanium hearing steel that has a large amount oftitanium, forming a rough surface. Lower titanium contents would,therefore, be more favorable.

Titanium carbide has a lower free energy of formation than the oxide orthe chloride and is apt to be less stable. The higher the titaniumcontent with a fixed carbon content, the more susceptible the base metalis to the interchange or interchange and etching reaction. The lower thetitanium content with the fixed carbon content, the less susceptible itis to the above type reactions and the greater it leans toward adeposition reaction. On the other hand, if the amount of titaniumdioxide present were in- In the case of the annealed specimens of bothlots, it was found that the primed ferrochrome was easily and readilyremoved from the surface of the specimens after chromizing. In the caseof the untreated specimens it was found that the primed ferrochromesintered and stuck to the specimens after chromizing and a great deal oflabor was required to remove the ferrochrome from the specimens. Even sothe resulting case was rough after the primed ferrochrome had beenremoved.

Based upon this experiment, it appears that hydrogen annealing incontact with alumina prior to chromizing produced the followingadvantages:

(1) The surface of the chromized work was very smooth.

(2) In a given time and at a given chromizing temperature, the thicknessof the chromized case was about 15- 25% greater in the instance of thespecimens which had first been annealed in hydrogen in contact withalumina.

(3) Where the specimens had not been annealed in hydrogen in contactwith alumina, etching occurred which not only impaired the surface ofthe specimens, but deposited a great deal more iron on the ferrochromeand thus more rapidly deteriorated the source of chromium. In the caseof the chromized specimens which had first been annealed in hydrogen incontact with alumina, a deposition reaction occurred without etching,thus tending to extend the life of the ferrochrome as well as improvethe surface appearance.

Example 2 Samples of low carbon plain carbon decarburized steel having acarbon content of 0.005% were degreased.

Samples of low carbon titanium stabilized steel, Lot 488772, as receivedfrom the steel mill were vapor degreased.

Some of the samples were chromized in contact with primed ferrochromeaccording to the procedure set forth above, and others of the specimenswere first annealed in a pack in contact with previously calcinedalumina in a stream of circulating hydrogen in a retort having a looselid at 1,800" P. for five hours, and then chromized. The hydrogenquality was as above defined and the circulation of hydrogen was asabove set forth.

In pack chromizing the previously treated and previously untreatedspecimens were held in contact with the primed ferrochrome at 1,775 F.for five hours in a closed retort protected from the air. Others of thepreviously treated and previously untreated specimens were gas chromizedaccording to the procedure set forth above at 1,775 F. for five andone-half hours.

The results of the two sets of experiments were as follows:

hydrogen and hydrogen bromide for five hours and then cooled to roomtemperature in the furnace. The parts were removed from the retorts andgiven a nitric acid support test, which demonstrated no appreciablechromizing had taken place. It is, however, possible that a minute filmof chromium might have diffused into the surfaces, but if so, it was notsufficient to give corrosion resistance under the nitric acid supporttest, either because of lack of thickness or lack of continuity or both.Some of the bolts from each lot were then contact chromized at 1775 F.for five hours. The results observed were as follows: I

The bolts which had been annealed in contact with alumina and nothydrogen had smooth chromized surfaces.

The bolts which had been annealed in hydrogen only without contact withalumina had smooth chromized surfaces.

Other bolts which had not undergone any annealing but were chromized inthe as-received condition had rough chromized surfaces.

This experiment indicates that under some conditions, annealing otherthan in contact with an oxide such as alumina can produce a smoothsurface after chromizing. One explanation may be that a slight diffusedchromium coating in the annealing accounts for this effect.

The indications are that a carbide formed on the surface due todiffusion of carbon from the core into the surface and ingress into theretort of hydrogen and chromium containing gases (from circulation ofthe hydrogen bromide through a source of chromium) and that thisresulted in formation of a chromium carbide surface due to the reductionof the chromium halide by the hydrogen. In

Contact Chromizing Gas Chromizing Percent sur- Gain or loss in facechromi- Gain or loss in Case Surface after weight after um (X-raySurface after weight after depth Identification chromizing chromizingfluorescence) chromizing chromlzing (mils) Untreated titanium stabilizedN on-uniform, with 3.0 gr./sq. ft 37.6 Slightly rough 3.6 gr./sq. ft 1.1

steel. local roughness. throughout. Annealed titanium stabilized Smooththroughout. +8.4 grJsq. ft 48. 1 Smooth surface +2.8 gr./sq. ft 1. 4

ste Untreated low carbon decar- Slight to moderate 1.1 grJsq. ft Shghflyough 7.99 grJsq. ft

burized steel. roughness throughthroughout.

ou Annealed low carbon decarbu- Smooth throughout.-. +8.1 grJsq. ftmooth throughout..- +1.3 gin/sq. ft

rized steel.

Example 3 Some titanium stabilized low carbon steel specimens anddecarburized low carbon steel specimens were annealed in a retort with asemi-tight cover in contact with previously calcined alumina in acirculating hydrogen atmosphere and other specimens from the same lotswere annealed under the same conditions in contact with hydrogen exceptthat no alumina was used.

All of the specimens were then contact chromized as in Example 2. Thosespecimens of titanium stabilized steel which were annealed in hydrogenin contact with alumina after chromizing give a smooth surface with acase of 1.9 mil thickness.

Those specimens of titanium stabilized steel which were annealed inhydrogen without any alumina in contact with them gave surfaces whichwere in some places rough and some places smooth, the case being 1.6mils thick.

Specimens of low carbon titanium stabilized steel which had simply beendegreased and were not annealed at all were chromized in the samebatches and they gave rough surfaces having a case thickness of 1.5mils.

Similar results were obtained for low carbon decarburized steel.

Example 4 Low carbon titanium stabilized steel bolts were in some casesplaced in powdered alumina in a retort with a semi-tight lid, and inother cases placed in an empty retort initially containing air andhaving a semi-tight lid.

Both retorts were placed in a gas chromizing furnace according to theSamuel and Bell patent above referred to and heated to 1775" F. in achromizing atmosphere of other words, instead of the hydrogen reducingthe oxide of alumina or chromic oxide in the pack technique, the samething occurred in gaseous chromizing. It will, of course, be understoodhowever that the ingress of these gases into the retorts was slightbecause of the semi-tight lid, as otherwise chromizing would have takenplace instead of annealing during the gaseous treatment.

Example 5 Low carbon titanium stabilized steel as above described in theform of sheet panels was solvent degreased, packed in a retort inpowdered chromic oxide (Cr O the retort having a semi-tight lid, andannealed in a furnace at 1,775 F. for three hours under circulatinghydrogen. The samples were then cooled in the furnace under hydrogen andremoved from the retort and separated from the chromic oxide. The panelsas treated above together with degreased untreated low carbon titaniumstabilized steel panels from the same lot were contact chromized underthe procedure above described at 1,775 F. for six hours. The resultswere as follows:

Case

depth in Identification Surface appearance mils Annealed in chromicoxide Smooth dull surface 1. 5 Untreated Rough bright surface 1. 2

1 1 Example 6 greased low carbon titanium stabilized steel from the samelot were annealed in hydrogen in contact with alumina for various timesand at various temperatures, the specimens being weighed before andafterward and a determination made of the Weight change. The resultswere as follows:

Weight Tempergain in ature, F. Hours mg./sq. it.

1, 670 3 8 to 44. 1, 775 3 96 to 112. 1, 875 3 170 to 307. 1, 775 6 180.

It is thus evident that a slight but substantial gain in weight of thespeciments results from annealing in hydrogen in contact with alumina.This is surprising in view of thermodynamic data which would lead one todoubt that aluminum could be reduced from alumina under theseconditions.

Example 7 Attempts were made to determine the nature of any addition tothe low carbon titanium stabilized steel panels by annealing in hydrogenin contact with aluminum oxide and in contact with chromic oxide.Untreated low carbon titanium stabilized steel panels, and specimensannealed in hydrogen in contact with calcined alumina and specimensannealed in hydrogen a contact with chromic oxide were subjected toX-ray fluorescence to determine the content of aluminum and of chromiumas the case may be.

Percent Percent aluminum chromium Appearance of Identification by weightby weight surface Untreated 0. 057 0.001 Steel-like. Annealed inhydrogen in 0.155 Aluminum-like.

alumina. Annealed in hydrogen 0. 042 Bluish.

in chromic oxide.

The liberated metal M forms a carbide on the surface of the specimen.

Example 8 Corrosion tests were made of degreased low carbon titaniumsteel specimens untreated, annealed in hydrogen as previously describedin contact with alumina and annealed in hydrogen as previously describedin contact with chromic oxide. Exposure was made outdoors in June in aresidential area for one week, which included two days of rain. Thespecimens on inspection showed the following:

Surface condition in per- Identification: centage of surface rustedAnnealed in hydrogen in alumina Annealed in hydrogen in chromic oxide 50Untreated 70 Annealed in hydrogen only 90 12 This experiment indicatesthat the thin diffused coatings of aluminum and chromium gave somedegree of protection against rusting as compared with the untreatedsteel.

Example 9 Degreased panel specimens of low carbon titanium stabilizedsteel as above described were annealed in hydrogen in calcined aluminaat 1,800 F. for five hours, as previously described. Half of each panelsurface was immersed in concentrated hydrochloric acid to remove thesurface metal. The panel was then rinsed, dried, buffed and reimmersedhalf way in concentrated hydrochloric acid and then again rinsed anddried.

The specimens thus prepared were then contact chromized according to theprevious procedure at 1,775 F. for five hours. The specimens Were then'cooled to room temperature and examined. Those areas where the surfaceof the annealed panel was removed by hydrochloric acid showed a roughnon-uniform chromized case. Those areas Where the panels had not beenimmersed in hydrochloric acid had a chromized case that was very smoothin appearance. This experiment confirms the fact that the annealing inhydrogen in alumina produces a surface effect which is beneficial to thesmoothness of the subsequently chromized case.

Example 10 A151 1070 steel was annealed in hydrogen in alumina accordingto the previous processing procedure at a temperature of 1,800 F. forfive hours. Annealed samples and unannealed samples were compared as tocarbon content:

Percent Unannealed 0.79'5 Annealed 0.345

This experiment demonstrates that the annealing in hydrogen in aluminais very effective to decarburize the steel, so that evidentlydecarburizing is one of the effects.

Example 11 Chromized surface layers were stripped from the panels of theprevious examples by dissolving the core in 30% by volume boiling nitricacid. The carbon contents of the chromized cases were then dedeterminedas follows: smooth, dull cases stripped from annealed specimens, 0.25%carbon; rough, bright cases stripped from untreated specimens, 0.05%.This indicates that a change in the distribution of carbon in the corehas occurred during annealing. Some of the carbon has diffused from thecore to the surface during annealing and as aluminum of chromium wasdeposited on the surface of the annealed specimen, aluminum carbide orchromium carbide for-med.

During the subsequent chromizing, it is evident that this carbon entersthe chromized layer as chromium carbide.

This carbide in the chromized layer, if it represents chromium carbideor less stable metal carbide than chromium carbide, will under someconditions adversely affect corrosion resistance, so that chromizedlayers containing it would be recommended more highly from thestandpoint of heat resistance than corrosion resistance as such. Morestable carbides, such as titanium carbide, calcium carbide and the like,do not have this disadvantage from the standpoint of corrosionresistance.

Annealing may be conducted in a reducing gas such as hydrogen, orcracked ammonia or a mixture of hydrogen, nitrogen and carbon monoxide,in contact with an Aluminum Tantalum Boron Thorium Calcium TitaniumColumbium (Niobium) Tungsten Chromium Uranium Molybdenum VanadiumSilicon Zirconium Strontium Example 12 Two lots of low carbontitanium-bearing steel were studied, Lot 45 944-2 and Lot 485 88-2. Theanalyses obtained from the steel manufacturer, which are believed to beladle analyses, and therefore subject to modification during subsequentprocessing, are as follows:

Percentage Element Lot 4-5944-2 Lot 4-8588-2 In order to check, ananalysis of certain elements was made based on the actual samples usedfor tests, with the following results:

Percentage Element Lot 4-5944-2 Lot 4-8588-2 Ti 0. 30 0. 36 C 0. 060 0.054 Mn 0. 33 0. 33

Panel samples of the two lots were solvent degreased and without anypreliminary annealing were chromized using gas chromizing technique asabove described at 1,825 F. for four hours and then furnace cooled. Thefollowing results were obtained:

Lot 4-5944-2 Lot 4-8588-2 Case depth (nitric acid etch), inch 0023 0017Weight gain or loss, g./sq. it +9. 3 -4. 2

Lot 4-5944-2 gave a smooth, dull chromized layer, and the mechanism ofchromizing was deposition and Lot 4-85882 gave a bright, rough chromizedlayer, and the mechanism of chromizing was etching and interchange.

Based upon the more accurate chemical analysis made, the ratio oftitanium to carbon for the two lots was as follows:

Lot 4-5944-2 5.1/1 Lot 485882 6.7/1

Example 13 The chromized specimens obtained in Example 12 were ground toexpose a bare edge, and the steel core was dissolved completely in aboiling solution of 30% by volume of nitric acid. The chromized caseswere then analyzed with the following result:

From the previous tests it is apparent that in the specimens which gavethe smooth, dull chromized cases, the titanium to carbon ratio was lowerin both the core and in the case while in the specimens which gave arough, bright chromized case the titanium to carbon ratio wassubstantially higher in both the core and the case.

Further experiments indicate that the ratio of stabilizer to carbon in astabilized steel has a powerful effect on the question of whether thesteel will form a smooth or a rough chromized layer. With titanium asthe stabilizer, the most powerful influence is exerted to form a smoothchromized layer by a titanium-to-carbon ratio of between 2.5 and 5.5 to1, and less effect but still powerful effect is exerted by the ratiobetween 5.5 and 5.75 to 1. The best ratio is between 3 and 5.5 to 1.

Other stabilizers instead of titanium should be used in the low ratiosto carbon as above set forth in order to get benefit in promotion ofsmooth deposition-type chromizing.

Example 14 A single lot of low carbon titanium bearing steel of atypical analysis as above set forth was received with a scaled oroxidized surface. After solvent degreasing, three sets of specimens wereprepared for chromizing as follows:

( 1) Untreated (2) The surface was pickled with hydrochloric acid untilthe scale was removed.

(3) The surface was sand blasted until the scale was removed.

All three specimens were chromized in contact with a primed pack offerrochrome as above set forth at 1,825 F. for four hours; The resultswere as follows:

(1) The specimen with the untreated surface had a dull and smoothchromized layer free from etching.

(2) The specimen with the pickled surface had a bright and roughchromized surface.

(3) The specimen with the sand blasted surface had a bright and roughchromized surface.

Both of the latter surfaces showed etching.

Example 15 Specimens which had been solvent degreased from low carbontitanium stabilized steel of Lots 459442 and 4-8588-2 were both annealedin hydrogen in contact with powdered titanium dioxide at 1,775 F. forfour hours. The specimens were cooled in the furnace in contact withhydrogen and then removed and chromized using the pack technique asabove set forth.

Lot 4-5944-2 showed the same results as previously set forth in Example12, with a dull, smooth chromized layer and a gain in weight indicatingdeposition.

Lot 4-8588-2 on the other hand now gave a smooth, dull chromized surfacewith greater case depth and a gain in weight, contrary to the previousbehavior in Example 12.

This shows that annealing in hydrogen and in titanium dioxide correctedthe tendency of Lot 4-8588-2 to produce rough chromizing.

It would seem that a slight reduction of titanium dioxide powder hastaken place producing titanium which has diffused into the surface ofthe core metal and reacted with carbon to form titanium carbide, andthat the titanium carbide has then functioned to produce the dull,

smooth chromized surface. The dullness can easily be corrected byburnishing.

Example 16 Corrosion tests were made of low carbon titanium stabilizedsteel which has been annealed in hydrogen and a compound such as chromicoxide, or titanium dioxide and then chromized to produce a smooth, dullchromized case. These tests indicate that as compared with smooth, dullchromized cases on low carbon titanium stabilized steel produced withoutpro-annealing and having a critical ratio of titanium to carbon withinthe range of 25:1 and 5.75:1, the corrosion resistance of the specimenswhich have been annealed prior to chromizing is not as good as thecorrosion resistance of the specimens which have not.

Example 17 Low carbon titanium bearing steel panels were placed inalumina in a retort containing a semi-tight cover. The retort was heatedto 1800 F. in hydrogen and held for 15 minutes. Afterwards it wasimmediately cooled to room temperature. The specimens were nextchromized along with low carbon titanium bearing steel specimens fromthe same lot which had not been previously annealed, the chromizingbeing carried out in the contact process at 1775 F. for six hours. Thespecimens which had previously been annealed were examined in comparisonwith the specimens which had previously been unannealed, and it wasfound that the previously annealed specimens had slightly smoothersurfaces.

FIGURE 1 illustrates in central vertical section an open coil annealingmechanism suitable for annealing according to the present invention,which embodies certain features present in Samuel and Bell US. Patent3,222,212, granted December 7, 1965, for Process of Chromizing. Themechanism will only be described insofar as it differs from that in thispatent.

An open coil 20 of steel sheet or strip is provided, wound with suitableopen spacing between turns, preferably in the range of 0.13 to 0.22inch. The coil rests on a base 21 supported by radial ribs 22 on asuitable refractory furnace base 23. The base 21 has a solid center 24which prevents unintended short circuiting of gas and a grill 25 onwhich the open coil rests on edge in vertical position as shown inFIGURE 1. Below the grill 25 the base has downwardly and inwardlydirecting bafiles 26 to funnel circulating gas into a center opening 27which is in line and immediately adjoining rotor 28 of a centrifugalblower or fan turning on shaft 30 which is suitably sealed against thesupporting base 23 to prevent leakage.

Surrounding the entire coil and base 21 and the fan is a bell closure 31resting on the base 23 and suitably sealed when in assembled position.Inlet pipe 32 introduces gas, suitably hydrogen, into the retort space33 inside the bell, and outlet pipe 34 draws ofi spent gas from theretort space.

In accordance with the present invention a quantity of granular oxide ofthe type above referred to is provided at 35, conveniently simply bypouring on top of the open coil before placing the bell to close thechamber, and the oxide is so finely divided preferably through 100 meshand most desirably through 325 mesh per linear inch that it becomesgasborne and circulates like a dust cloud under the action of the pumpor blower through the open coil in contact with the steel sheet or stripand in endless circulation. Very effective annealing according to theinvention is obtained by this means.

Surrounding the bell 31 is placed a removable bell furnace 36 havingelectric heating elements 37 which when in place is capable of raisingthe entire work and the retort in which it is contained, closed from theair, to a suitable annealing temperature in the range from 1,500 to2,000 F.

In the preferred embodiment of the furnace of FIG- URE 1 the open coilfrom the previous heat is removed,

removing the furnace 36 and the bell 31. Then the open coil forannealing in the next heat is introduced and the oxide in powdered formis placed on it, after which the retort bell 31 and the bell furnace areapplied, the air is swept out of the retort suitably by nitrogen. Beforereaching an elevated temperature hydrogen is introduced to fill theretort 33. The fan blower or pump 28 is then rotated in order tocirculate the gas and the cloud of oxide particles in contact with thework for a time of at least one hour at temperature.

Since hydrogen is very light, it is preferable to include a dense gas inthe gas mixture. Argon is a desirable gas to include with hydrogen or amixture of hydrogen with a small amount of carbon dioxide or carbondioxide and nitrogen. A desirable gas composition by volume is 50percent hydrogen and 50 percent argon.

In case powder pack annealing is desired to be employed, the spacesbetween the open coils can simply be filled by sifting oxide particlesinto the space, and the blower need not be operated, simply maintainingstatic conditions and passing gas through the retort. As an alternativethe coil for powder pack annealing can be coated with oxide particles,and in this case the turns can be close together if desired althoughthey are preferably spaced in order to aid in the reduction reaction.

The annealing can be carried out continuously as shown in FIGURE 2.

In this form, work such as a strip or sheet 20' is unwound from one roll40 onto another roll 41. The work. passes into an annealing furnace 42through a series of slits or openings 43 sealed in any desired manner. Avestibule chamber 44 is provided at the entering end having an inlet gasport 45 and an outlet gas port 46 to remove air, and a vestibule chamber44 is provided at the outlet and having an inlet gas pipe 45' and anoutlet gas opening 46 to exclude air. A suitable protecting gas, whichmay be hydrogen, can be introduced in the vestibules and burned at theslits 43 adjoining the atmosphere.

The work 20' then passes into an annealing chamber 47 ,heated, forexample, by electric heating elements 48 to a suitable temperature, forexample, 1,500 to 2,000 F. A mixture of particles of suitable oxide 50,for example, alumina, is introduced from a suitable dry feeder 51 into astream of gas, for example, hydrogen passing through pipe 52 into theannealing chamber and spent gas and oxide particles are removed througha pipe 53.

In operation the work passes through the annealing furnace and throughthe annealing chamber and in the annealing chamber comes in contact withgas-blown particles of the selected oxide as above mentioned in thepresence of a reducing gas at a temperature of 1,500 to 2,000 F. In thiscase annealing can be accomplished in a time of 10 or preferably 15minutes.

In view of my invention and disclosure, variations and modifications tomeet individual whim or particular need will doubtless become evident toothers skilled in the art to obtain all or part of the benefits of myinvention without copying the process, apparatus and product shown.

Having thus described my invention what I claim as new and desire tosecure by Letters Patent is:

1. A process of producing a chromized layer on a ferrous metal, whichcomprises providing a wrought ferrous metal of the class consisting ofsteel of carbon content between 0.001 and 0.005% and stabilized steel ofcarbon content between 0.001 and 0.20% and having a stabilizing elementof the class listed below in a ratio to carbon as listed below:

Titanium Between 4 and 12 to 1 Columbium Between 8 and 20 to 1 VanadiumBetween 4 and 12 to 1 Tungsten Between 15 and 50 to 1 heating said metalto a temperature of 1,500 to 2,000 F. in contact with a granular oxideof an element of the class consisting of aluminum, boron, calcium,columbium,

17 chromium, molybdenum, silicon, strontium, tantalum, titanium,thorium, tungsten, uranium, vanadium, and zirconium in a reducingatmosphere for a time of at least ten minutes, separating said ferrousmetal from said oxide and said reducing atmosphere and then heating saidferrous metal to a temperature between 1,550 and 2,000 F. for a time ofat least one hour in an atmosphere protected from the air and containinga halogen chromizing catalyst in the presence of a source of chromium todeposit chromium on the ferrous metal, whereby a preferential tendencyto make a smooth chromized layer is obtained.

2. A process of claim 1, in which the reducing gas is ahydrogen-containing gas.

3. A process of claim 1, in which the oxide is alumina.

4. A process of claim 1, in which the oxide is titania.

5. A process of claim 1, in which the oxide is chromic oxide.

6. A process of claim 1, which comprises circulating said oxide in theatmosphere in contact with the ferrous metal.

7. A process of claim 1, which comprises during the chromizingmaintaining a granular source of chromium in contact with the ferrousmetal being chromized.

8. A process of claim 1, which comprises during the chromizingcirculating a halogen-containing gas through a granular source ofchromium and then in contact with the ferrous metal.

9. A process of claim 1, in which the ratio of the element of the classto carbon is as follows:

Titanium Between 2.5 and 5.75 to 1 Columbium Between 4.9 and 11.2 to 1Vanadium Between 2.5 and 5.75 to 1 Tungsten Between 9.1 and 20.8 to 1.

10. A process of claim 9, in which the ratio of the element of the classto carbon is as follows;

18 Titanium Between 3 and 5.5 to 1 Columbium Between 5.8 and 10.7 to 1Vanadium Between 3 and 5.5 to.1 Tungsten Between 10.9 and 21.1 to 1.

11. A process of claim 9, in which the reducing gas is ahydrogen-containing gas.

12. A process of claim 9, in which the oxide is alumina.

13. A process of claim 9, in which the oxide is titania.

14. A process of claim 9, in which the oxide is chromic oxide. 5 15. Aprocess of claim 10, which comprises blowing the oxide against theferrous metal.

16. A process of claim 9, in which the steel is in contact with a sourceof chromium during chromizing.

17. A process of claim 9, which comprises circulating thehalogen-containing gas through a source of chromium and in contact withthe steel during chromizing.

References Cited UNITED STATES PATENTS 1,853,369 4/1932 Marshall 117-222,032,694 3/1936 Gertler 117-16 2,141,640 12/1938 Cooper 117-222,157,594 5/1939 Cooper 117-22 2,255,482 9/1941 Daeves et a1. 117-222,899,332 8/1959 Samuel 117-22 2,935,420 5/1960 Linden 117-22 X3,028,261 4/1962 Wachtell et al 117-22 X 3,178,308 4/1965 Oxley et a1.117-107.2 X 3,183,113 5/1965 Gemmer 117-21 3,208,870 9/1965 Criss117-118 WILLIAM D. MARTIN, Primary Examiner.

PAUL ATTAGUILE, Assistant Examiner.

US. Cl. X.R. 117-50, 71, 118

